The present invention relates to an apparatus for removing sheets, one-by-one, in a removal direction from the top of a stack of sheets, which comprises a support for the stack of sheets, removal means disposed a short distance above the stack for attracting and removing, in the removal direction, the top sheet of a stack of sheets lying on the support, and side-blowing means for blowing air, looking in the removal direction of the sheets, from the side in the direction of the stack.
An apparatus of this kind is known from U.S. Pat. No. 4,787,618. This patent describes an apparatus wherein the removal means comprise a suction means against which the top sheet of the stack is firmly sucked so as to be removed on transport by said suction means, and wherein in addition a side-blowing means introduces air into the stack of sheets in the event of sheets sticking to one another and in the event the top sheet is not brought into contact with the suction means at the correct time.
The range of sheets used as receiving material in the current generation of printing machines is considerable and it is expected that it will increase even more in future. In these conditions, a reliable supply of this broad range of receiving materials, varying from thin to thick receiving material and from smooth to stiff receiving material, is subject to increasingly stringent requirements. Especially in the case of fast productive printing machines, it is desirable to minimise feed faults, particularly the feed of double sheets or the failure of sheet feed, in order that the time elapsing between two feed faults in high-speed machines can be made acceptably short. One and the same malfunction frequency (number of faults per total number of feed cycles) will occur, for example, just once a day in the case of a slow and less productive printing machine, but, for example, once per hour in the case of a high-speed productive printing machine, the latter being unacceptable.
In order to obtain good separation between just the top sheet of a stack and the rest of the stack it is known, from European Patent Application 0 223 502, to direct a number of differently directed air jets onto the front of the stack of sheets in a direction which is opposed to the direction in which the sheets are removed from the stack one-by-one. These air jets serve to blow the top sheets of the stack loose from one another and move them upwards in the direction of a suction conveyor belt disposed above the stack and in order to create a positive pressure in the area between the top sheet sucked against the suction conveyor belt and the rest of the stack to ensure that on the removal of the top sheet by the suction conveyor belt the following sheets of the stack are retained by pressing them down.
One disadvantage of this latter known apparatus is that, necessarily, the transport path provided at the front of the stack for the removal of the top sheet, as considered in height, limits the space for providing the blowing means. The blowing means must in fact be disposed at an ample distance beneath the suction conveyor belt to offer sufficient free access between the belt and the blowing means for the undisturbed removal of sheets from the stack. This limits the freedom for achieving optimal blowing geometry.
An object of the present invention is to provide an apparatus which solves the above disadvantages.
To this end, according to the present invention, the side-blowing means generates at least two differently directed air flows, a first air flow substantially parallel to the plane in which the sheets are lying and at least directed on the top sheets of the stack and the space thereabove, and a second air flow which, with respect to the first air flow, is directed obliquely upwards in the direction of a sheet attracted by the removal means. As a result, when the top sheets of the stack which have been blown loose and brought up by the first air flow have come into the range of the suction conveyor belt, the sheet situated beneath the top sheet attracted by the suction conveyor belt is effectively loosened from said top sheet because of the creation of a flat air stream beneath the attracted sheet which, comparable to the fluttering of a flag in the wind, shakes any attached sheet loose. Another effect is that the blowing means can occupy a fixed orientation with respect to the stack, by accommodating them in a slidable side guide, so that the operation is independent of the sheet format.
Preferably, front-blowing means are provided to blow air in a direction opposed to the removal direction between a sheet attracted by the removal means and the rest of the stack. As a result, air can readily be blown into the space beneath the top sheet as created by the side-blowing means, so that sheets situated beneath the top sheet can be pressed down in order to retain the same on removal of the top sheet.
The side-blowing means can also generate at least a third air flow, directed on sheets situated beneath the top sheets of the stack. The effect of this is that sheets which cannot easily be blown loose and lifted by the first air flow, for example because they are too heavy and/or curved with the convex side lying at the top on the support, are brought within range of the first air flow and the attracting action of the removal means.